The proposed experiments will deal with the chronic production of physiologic serum concentrations of testosterone, dihydrotestosterone and estradiol in castrated rats and the effects of such treatment on the secretion of LH and FSH. Comparable studies will be conducted in intact animals and in hypophysectomized, castrated rats on both sexes. The serum of these later groups will be assayed (by radioimmunoassay) for circulating LH/FSH releasing factor in the hope of demonstrating the effects of steroids on the hypothalamic secretion of the releasing factor.